I have just been testing a quintifilar 50:75 ohm UNUN that I wound
according to instructions from Sevick's book, figure 7-8(a). If I can get
two of them that work well, I plan to use them to match my 75-ohm CATV
feedline to 50-ohm everything else, without using half-wavelength multiples
as I do now.
Using an MFJ 259B with a 75-ohm resistor on the 75-ohm port, the
performance of the UNUN is quite good from 1.7 to about 11 MHz, with
measured R of 48-50 ohms and X typically 5 ohms or less. Above that
frequency, however, the UNUN shows fairly rapidly increasing R and varying
X (for example, at 19-21 MHz R=80-90 and X=0, for an SWR of 1.7:1, and at
28.7 MHz R=62 and X=46 (all X signs unknown)). Clearly this is
unsatisfactory, but I really don't know what to do now. Any ideas?
73, Pete N4ZR